The present invention relates to a method for treating a bed of particulate material which is supported by a gas distribution bottom, utilizing a treatment gas which via ducts is conducted in sectionalized manner to and directed up through the gas distribution bottom and the bed of material from one or several underlying compartments. The invention relates also to an apparatus for carrying out the method according to the invention.
Within the industrial sector there are numerous examples of apparatuses which comprise a gas distribution bottom. As nonlimiting examples hereof can be mentioned fluidized bed reactors, chemical reactors, drying apparatus, gas-solid heat exchangers and others.
Essentially, the functions of the gas distribution bottom is to support the bed of material and to distribute the treatment and fluidizing gas uniformly across the entire bed. The construction of the gas distribution bottom is also of importance both for the physical and chemical efficiency of the bed. To-day it is a generally recognized and reluctantly acknowledged fact that a relatively high pressure drop across the gas distribution bottom is required to ensure uniform distribution of the gas across the entire bottom, since improper distribution of the gas flow will often lead to poor gas-solid contact and formation of tunnels. Often, a gas distribution bottom is characterized by the relationship between the pressure drop across the gas distribution bottom and the pressure drop across the bed. In technical literature, it is typically recommended that the gas distribution bottom be configured so that this relationship will be 0.40 or higher, i.e. the pressure drop across the bottom is at least 40% of that across the bed. However, this relatively high pressure drop across the gas distribution bottom entails an excessively high energy consumption of the fan installation which propels the treatment gas through the apparatus.
An example of an apparatus which comprises a gas distribution bottom is a grate cooler for cooling, for example, cement clinker. In such a cooler the primary aim is to achieve a favourable degree of heat exchange between the clinker and the cooling gas so that essentially all the thermal energy contained in the hot clinker can be returned to the kiln system in the cooling gas, while, at the same time, the clinker is discharged from the cooler at a temperature which is very close to the ambient temperature. It is a precondition for achieving a favourable degree of heat exchange that the cooling gas flow through the clinker is well-defined.
In connection with the cooling of cement clinker which is discharged from a kiln installed ahead of the cooler it has, however, emerged that the clinker is not always uniformly distributed on the cooler grate. Instead, there is a tendency towards the clinker being distributed so that the larger clinker lumps are predominantly located at one side of the cooler, whereas the finer clinker lumps are located at the other side. Also, the thickness of the clinker bed may exhibit variations both longitudinally and transversely through the cooler. Since it is easier for the cooling gas to penetrate a bed of larger clinker lumps and/or a thinner bed as compared to penetrating a bed of finer clinker lumps and/or a thicker bed, and since, quite naturally, the cooling gas will always follow the route of least resistance, any such uneven distribution of clinker often entails that the finer clinker material is not sufficiently cooled, hence causing hot zones, so-called "red rivers", to be formed in the cooler. Such uneven distribution of the clinker may also entail that the cooling gas in the areas where it encounters least resistance will simply blow the material away and form tunnels, through which the cooling gas will escape without any noteworthy exchange of heat with the clinker material. Therefore, optimum efficiency of a cooler operating under such conditions cannot be achieved.
In order to reduce the importance of the uneven penetrability of the clinker bed of the cooler gas and to ensure a more evenly distributed cooling gas flow across the entire surface of the grate, it has been proposed that the grate proper be provided in such a way that the grate itself will put up great resistance to the penetration of the cooling gas. However, this solution entails a major pressure loss across the grate, involving substantial costs for the erection and operation of the fan installation. At the same time, it does not eliminate the problems in terms of tunnel formations.
From EP-A-0 442 129 is known a method and a grate cooler by means of which the aforementioned problem is claimed to be minimized by feeding additional cooling gas in pulses to the areas of the bed in which the temperature is higher than in the surrounding areas of bed, whereby the first-mentioned areas of bed are cooled further and subjected also to agitation. A distinct disadvantage of this known solution is the relatively expensive and complicated manner in which the control operation for the additional cooling gas supply is carried out. Controlling involves that the temperature of the entire surface area of the material bed is measured and recorded in order to establish a temperature profile which, via a calculating and controlling unit, forms the overall basis for controlling a number of valves which admit and shut off, respectively, the supply of additional cooling gas to nozzles which are fitted under the grate in a structured pattern. Also, the agitation of the material bed may have a negative effect on the efficiency of the cooler.
A second example of an apparatus comprising a gas distribution bottom is a fluidized bed kiln which is used e.g. in heat and power plants. In a fluidized bed the primary aim is to ensure efficient combustion of the input fuel under stable and optimum operating conditions. In this context, it is a precondition that the fluidizing gas is evenly distributed across the entire bed.
In the fluidized bed kiln there are known problems in terms of tunnel formations similar to those described above in connection with the cooler example. In the fluidized bed kiln the problem is also believed to be attributable to the fact that the thickness of bed is not uniform, thereby causing the fluidizing gas to penetrate, with a self-energizing effect, the bed at the point of least thickness and, therefore, the point of least resistance. In order to minimize the problem and to achieve a more uniform distribution of the fluidizing gas, the gas distribution bottom has been provided in similar manner as done in the clinker cooler, so that it put up great resistance to the penetration of the fluidizing gas. However, it has been ascertained that, nor in fluidized bed kilns, has this solution led to elimination of the problem in terms of tunnel formations.